In recent years, increasing efficiency of power generation plants is in progress from a viewpoint of reducing emission of carbon dioxide into the air. Accordingly, increasing efficiency of a steam turbine and a gas turbine included in a thermal power generation plant is demanded. Besides, increasing efficiency of a CO2 turbine capable of being installed in the thermal power generation plant is also demanded.
It is effective to increase an inlet temperature of working fluid introduced to a turbine to increase the efficiency in the above-described each turbine. For example, in the steam turbine, an operation under a condition where a temperature of steam being the working fluid is 650° C. or more, further approximately 700° C. is expected in the future. There is a tendency that the inlet temperature of the introduced working fluid increases also in the gas turbine and the CO2 turbine.
Conventionally, ferritic heat resistant steel or the like has been used for turbine components exposed to the temperature at approximately 600° C. However, there is a problem in heat resistance to constitute the turbine components exposed to the high-temperature working fluid as stated above with the ferritic heat resistant steel. Accordingly, the turbine components exposed to the high-temperature working fluid are constituted by austenitic heat resistant steel, an Ni-based alloy, a Co-based alloy, and so on. Among them, a service temperature of the austenitic heat resistant steel is higher than the ferritic heat resistant steel for approximately 50° C., and a material cost of the austenitic heat resistant steel is approximately one-third of the Ni-based alloy. Accordingly, it is possible to suppress a manufacturing cost and to enable high efficiency by using the austenitic heat resistant steel.
There has been a lot of development regarding the austenitic heat resistant steel mainly focusing on improvement in high-temperature strength. However, the austenitic heat resistant steel has a characteristic where a linear expansion coefficient is high, and there is a problem that repeated thermal stress due to start/stop operation is excessively generated when the austenitic heat resistant steel is considered to be applied to a valve and an inner casing of a steam turbine. Besides, it is known that crack sensitivity of the austenitic heat resistant steel at a welding time is high, and manufacturing defects are easy to occur when the valve and the inner casing are manufactured by welding. In publicly known austenitic heat resistant steel such as Alloy286, improvement in high-temperature creep strength is attempted by using an intermetallic compound as a precipitation strengthening phase. However, there is not proposed heat resistant steel securing excellent weldability while having a low linear expansion coefficient.
Thermal expansion characteristics and weldability of a material are important factors in designing a high-temperature structural material. However, in conventional austenitic heat resistant steel, it was difficult to secure excellent weldability while having a low linear expansion coefficient.